A decrease in renal blood flow, especially if severe and prolonged, may lead to acute renal failure. The factors contributing to renal insufficiency after a period of ischemia are poorly understood. Decreased glomerular filtration pressure, interstitial edema, tubular leakiness, and tubular obstruction have all been suggested as factors contributing to renal failure. Severe renal ischemia will be produced in anesthetized rats by complete renal artery occlusion of 1 or 2 hours duration. Kidney function will be studied before occlusion, immediately after release of the occlusion, and 24 hours later. Renal clearance and micropuncture techniques will be used. Glomerular capillary, peritubular capillary, tubular, and subcapsular pressures will be measured by servo null techniques. Single nephron filtration rate will be measured with inulin. Tubular permeability to inulin will be tested by microinjection of inulin-C14. Fluid reabsorption will be measured with the split-droplet technique. Blood flow will be measured by direct cannulation of the renal vein. The data obtained should explain the reasons for renal insufficiency after an acute ischemic insult.